Pursuant to 35 USC xc2xa7 119, this application claims the benefit of Singapore Patent Application No. 200204608-4 filed Jul. 31, 2002.
This invention relates to a method and apparatus for cleaning of solid surfaces, and more particularly a method and apparatus of removing micron or sub-micron size particulates on solid surfaces with the use of indirect laser irradiation.
Laser cleaning is a relatively new technique for cleaning surfaces and may be divided into two categories, dry laser cleaning and steam laser cleaning. The following prior art discloses the details of how these laser cleaning methods are applied to remove particles on the solid surfaces: U.S. Pat. Nos. 4,980,536, 5,151,135, 5,643,367, Re35,981 and 6,291,796 B1. And U.S. published application No. 2001/0,011,545 A1.
In U.S. Pat. No. 4,980,536, small particles are removed from the solid surface like that of a silicon (Si) mask by bombardment of high-powered laser pulse. U.S. Pat. No. 5,151,135, teaches the use of short lower energy pulses of ultraviolet (UV) laser radiation on solid surfaces while U.S. Pat. No. 5,643,367, provides two stages of cleaning solid surfaces, such as a mould for making glass articles. The first stage combines the action of washing baths and ultrasound while the second stage uses a laser to impact the surface region. This second stage supplements the first stage in the cleaning process by removing left over residue. U.S. Pat. No. Re 35,981 also discloses a system for cleaning moulds, by direct laser irradiation on the surface within the mould. U.S. published application No. 2001/0011545 A1 relates to the use of laser irradiation on surfaces of semiconductor wafers or the like, where the surface is wetted with a liquid before irradiating the surface with laser with sufficient photon energy to remove the liquid and the particles on the surface. U.S. Pat. No. 6,291,796 B1, discloses an apparatus using dry laser cleaning.
In related literature, xe2x80x9cLaser-cleaning techniques for removal of surface particlesxe2x80x9d (pp. 3515-3523) J. Appl. Phys. 71(7), 1992 by A. C. Tam, W. P. Leung, W. Zapka and W. Ziemlich, the action of dry laser cleaning and steam laser cleaning is disclosed. xe2x80x9cLaser removal of particles from magnetic head slidersxe2x80x9d (pp499-504) J. Appl. Phys., 80(1), 1996 by Y. F. Lu, W. D. Song, M. H. Hong, B. S. Teo, T. C. Chong and T. S. Low teaches the application of dry laser cleaning on magnetic head sliders. xe2x80x9cThe modeling of excimer laser particle removal from hydrophilic silicon surfacesxe2x80x9d (pp3618-3627) J. Appl. Phys., 87(8), 2000 by X. Wu, E. Sacher and M. Meunier provides theoretical models in laser cleaning.
All the above-referenced prior art rely on pulsed laser heating of solid surfaces with or without the presence of a thin liquid coating. In dry laser cleaning, contaminants can be removed from solid surfaces by laser-induced fast thermal expansion of contaminants and/or solid surfaces or laser ablation of contaminants. In steam laser cleaning, contaminants are removed by laser-induced explosive vaporization of the liquid coating on the solid surfaces.
In both steam and dry laser cleaning, contaminants are removed from solid surfaces by laser irradiation directly onto the surfaces with or without liquid film assistance. Since most solid surfaces tend to absorb laser irradiation, the absorption induces high temperatures in the solid surfaces as the laser irradiates the solid surfaces. One problem associated with this technique is damage to the surfaces, such as oxidation, melting, ablation, crack and stress generation in the top layer of a solid substrate, and other changes of physical and chemical properties. Such changes affect the performance of the solid substrate. As mentioned in section xe2x80x9cA. Damage thresholdxe2x80x9d in the publication xe2x80x9cLaser removal of particles from magnetic head slidersxe2x80x9d, it was found that there is a damage threshold beyond which the substrate would experience micro-cracks after a number of heating and cooling cycles. Such substrates would have lost more than 20% of their performance as compared to a performance when they are not damaged. Therefore, it is highly desirable to have a method for laser cleaning without heating solid surfaces directly which alters the surface properties of the substrates.
The present invention is a method of cleaning solid surfaces through the use of an apparatus for the method, which focus a laser beam at a point close to the surfaces to be cleaned in a liquid medium.
In the first aspect of the present invention, an apparatus is provided, for cleaning a surface. The apparatus has a laser beam source, an optical system for focusing a laser beam from the laser beam source and a liquid in which the surface to be cleaned is immersed. The optical system is arranged in the apparatus such that the laser beam is focused at a point in the liquid, which is in close proximity to the surface to be cleaned but not directly at the surface. The apparatus also has a stage for securing the surface such that the surface can be immersed into the liquid and removed from the liquid when it is mounted on the stage. The stage can also enable the surface to be immersed fully or partially in the liquid. The stage can also move the surface horizontally in the liquid and the rate of movement of the stage is determined by the type of contaminant and/or the amount of contaminant to be removed.
The apparatus as provided in this invention can be used to clean a surface of a silicon (Si) substrate, a disk or a magnetic head slider.
In a further embodiment, the apparatus includes a filter system for filtering particles in liquid and pouring new clean mixture, liquid into container.
In yet a further preferred embodiment, the laser beam creates bubbles when it is focused at a point in the liquid. The bubbles in turn generate a laser-induced liquid jet and shock wave at the interface between bubble and solid surface to be cleaned at the moment of collapse. It is preferred that the laser beam has a pulse duration ranging from 1 nanosecond to 100 microseconds and is focused close to the surface to be cleaned. The laser source can be a YAG laser, an excimer laser, or CO2 laser.
Preferably, the laser source provides a laser beam with a laser fluence in the range of 0.5 J/cm2 to 100 J/cm2.
Preferably, the laser beam has a frequency range from 1 Hz to 10 kHz.
Preferably, the laser beam has a pulse number from 1 to 10000 for cleaning a same position.
Preferably the laser beam has a wavelength ranging from 157 nm to 10.6 xcexcm.
In another embodiment of the invention, a gas blower is provided to blow stray liquid spray towards the optical system and to dry the cleaned surface.
It is preferred that the apparatus has a container to hold the liquid.
In a preferred embodiment, the liquid enables the easy of the cleaning in reducing the adhesion of the particles to the substrate surface.
Preferably, the liquid in the container is pure water.
Preferably, the liquid is a mixture of water and various types of solvent.
The types of solvent can be alcohol, acetone, aromatics, ethers, ketones, alkanes, halogenated hydrocarbons or commercial washing solution which helps to reduce adhesion forces of particles on the surface to be cleaned and enhance cleaning efficiency.
In a second aspect of the invention, a method for cleaning surfaces is provided, the method includes securing a surface to be cleaned in a liquid; focusing a laser beam at a point in the liquid to generate a laser-induced liquid jet and a shock wave; and positioning the point of focus of the laser beam in close proximity to the surface to be cleaned such that the laser-induced liquid jet and shock waves clean the surface.
In a preferred embodiment, bubbles are generated in the liquid about the point of focus of the laser beam.
In yet another preferred embodiment, the bubbles collapse to generate the liquid jet and shock wave for cleaning the surface.
In one embodiment of this method, the laser beam has a pulse duration in the range from 1 nanosecond to 100 microseconds.
Preferably the laser beam is focused within a range from 0.1 mm to about 10 cm from the surface to be cleaned.
Preferably, the laser beam can be a YAG laser, an excimer laser, or CO2 laser.
Preferably, the laser beam has a laser fluence in the range of 0.5 J/cm2 to 100 J/cm2.
Preferably, the laser beam has a frequency range from 1 Hz to 10 kHz . . .
Preferably, the laser beam has a pulse number from 1 to 10000 for cleaning a same position.
Preferably the laser beam has a wavelength ranging from 157 nm to 10.6 xcexcm.
Preferably, the method includes blowing stray liquid spray towards the optical system and drying the cleaned surface. This is achieved by using a gas blower.
Preferably, the method further includes filtering of particles in the liquid.
In another preferred embodiment, the securing of the surface to be cleaned in the liquid is made possible by the use of a stage onto which the surface is secured. This enables the surface to move within the liquid as well as into and out of the liquid. The surface can also be immersed fully or partially in the liquid by the stage.
In another preferred embodiment, the method includes a container for holding the liquid.
Preferably the liquid is pure water.
Preferably, the liquid is a mixture of water and various types of solvent.
The types of solvent includes alcohol, acetone, aromatics, esthers, ketones, alkanes, halogenated hydrocarbons and commercial washing solutions which helps to reduce adhesion forces of particles on the surface to be cleaned and enhance cleaning efficiency.
It will be appreciated that the method of the present invention is different from dry and steam laser cleaning disclosed in the prior art as discussed in the above paragraphs. As a high power laser beam is focused into liquid, it creates a lot of bubbles there. When a solid substrate is inserted into the liquid close to the focal point where the laser beam impact the liquid, bubbles are formed at the region of the focus point of the laser beam. The interaction between the bubbles and the substrate results in bubble collapse. At the moment of bubble collapse, a shock wave is generated and a high-speed liquid jet is formed almost simultaneously. These high-speed liquid jet and shock wave create a large force causing contaminants particles to dislodge from the solid surface when it shoots towards the surface. The present invention is based on these laser-induced liquid jet and shock wave to clean solid surfaces. Since laser does not irradiate onto solid surfaces, surface damage due to thermal effects induced by laser irradiation directly onto solid surfaces in dry and steam laser cleaning can be avoided by the present invention.
With the present invention, the surface damage due to thermal effects induced by laser irradiation directly onto solid surface in prior laser cleaning methods can be avoided and contaminants especially particles and organic contaminants on solid surfaces can be effectively removed by laser-induced liquid jet and shock wave.